This invention relates to the cooling of a direct current type dynamoelectric machine and in particular it relates to an improved means for cooling a commutator type direct current dynamoelectric machine.
In an AC machine, air or other cooling gas can be introduced into the machine at either or both ends of the machine and passed radially outwards through cooling passages for discharge or recirculation. In a direct current or DC machine with a relatively large commutator at one end, it is very difficult and undesirable to bring air (or other cooling gas) equally from both ends of the rotor. Consequently it has been the practice to introduce the air at the end of the machine remote from the commutator and to discharge it at the commutator end. Carbon dust from the commutator brushes is thus conveniently carried away from the machine. In these prior machines air is introduced into axially extending passages in the rotor spider and travels radially outwards through spaced ducts in the rotor core to the air gap between the stator and the rotor. The air travels, in part, through the air gap to the commutator end where it enters a collecting chamber which communicates with an exhaust. A portion of the air may travel from the air gap radially outwards through interpolar spaces and from there both axially in the interpolar spaces and outwards through openings into a peripheral axial passage. Both the interpolar spaces and the peripheral spaces communicate with the collecting chamber at the commutator end. There are pressure drops along the rotor passages and along the air gap path, as well as in the peripheral passage. Consequently the flow of air is not equal along the axis, and there tends to be regions of higher temperatures. It is, of course, desirable to have uniform temperatures axially along the rotor and stator cores.
Canadian Patent No. 1,024,576 issued on Nov. 7, 1978 to Anil K. Mishra, describes an apparatus for reducing this problem. The air passage which extends between the spider arms of the rotor is tapered. This is intended to provide a constant static pressure along the axially extending rotor passage and thus create a more uniform radial flow through the rotor core by reducing the so-called "manifold effect" at the inlet side of the radial passages. However it has been found that this occurs only when the cross-sectional area of the tapered passage is relatively small. The path of cooling air is along this axial passage, radially outwards through ducts in the rotor core, and then along the air gap. The cross-sectional area of the rotor passages is normally large in comparison to the cross-sectional area of the air gap flow path, and the more severe manifold effect occurs in the air gap. The non-uniformity of static pressure in the air gap is particularly severe in more recent machine designs which tend to have a relatively long axial dimension. Consequently the use of a tapered air passage in the rotor is of limited value because it provides desirable results only when the rotor air passage is relatively small or conversely the air gap is unusually large. It is, of course, difficult to increase the size or magnitude of the air gap without a severe reduction in electromagnetic performance. Some of the air flow is outwards from the air gap through the stator and through the peripheral passage. The peripheral passage is usually of relatively large cross-sectional area and there is a very much smaller manifold effect, but this may also be reduced by the present invention.
The present invention seeks to reduce the manifold effect in the air gap by reducing significantly the maximum axial velocity of the air in the air gap.